For certain workpieces or structural components it is often necessary to determine if internal stresses are present therein, and what magnitude and direction these comprise. In that regard, the term internal stresses means stresses in a structural component that are effective without external mechanical loading and that are subject to a spatially homogeneous and temporally constant temperature field.
For measuring such internal stresses, there are various different testing methods with which the internal stress condition on the surface and in the interior of the material can be determined. In the range near the surface up to a depth of several millimeters, the bore hole method is often utilized. Thereby, a hole is bored at a certain location of the structural component at which the internal stresses are to be determined, wherein the diameter and the bore depth of the hole is based on the test sample thickness. Through this bore hole, a portion of the internal stresses present in the material is released, whereby measurable deformations arise in the vicinity of the bore hole rim.
By measuring these deformations in the region of the bore hole, a conclusion can then be reached about the effective internal stresses. For that, the strains in three different directions must be determined, for example by applying three strain gages that are grouped around the bore hole. In this method, however, only relatively small strain gages can be used, because the substantial analogy between the differential strain and the triggered internal stress portions exists only in the direct vicinity of the bore hole.
Such a method and a strain gage rosette used therefor is known from the DE-OS 25 58 768. Therein, the strain gage rosette consists of a carrier film on which three measuring grids are arranged at the angular positions of 0°, 45° and 90° around a central middle point or center point. In the internal stress measurement according to the bore hole method, the difficulty is that the workpiece is to be bored exactly in the middle of the glued-on rosette. Namely, with a bore hole that is not centrally applied, the strain measuring grids, which are then arranged at different radial distance, detect a spacing distance dependent strain, which then lead to measurement errors. Therefore, the strain gage rosette is glued-on with its center point exactly centered on a circuit board, whereby a centering bushing is soldered-on in the center of the circuit board. In that regard, a central bore hole is provided in the centering bushing, and a centering pin is inserted in the central bore hole. The strain gage rosette is then glued onto the workpiece to be tested.
For the exact introduction of the bore hole, then a special drilling or boring apparatus of a U-shaped metal bail with a drill or borer guide is arranged over the strain gage rosette, and is applied or set-on exactly centered to the strain gage rosette by a centering pin that is inserted in the centering bushing. Then, after the centering, the centering pin is removed out of the drilling or boring apparatus, and the shaft of a drilling or boring machine is inserted into the drilling or boring apparatus, in order to introduce into the workpiece a very exact central bore hole between the glued-on measuring grids. The strains that are thereby triggered during the drilling or boring process are then converted into electrical signals by the is strain gage grids, and then the internal stresses are calculable from the electrical signals. Because a substantial analogy between the differential strain and the triggered internal stress portions exists only in the direct vicinity of the bore hole, the strain gages grouped around the bore hole must be relatively small. Therefore such strain gage rosettes often have only diameters of approximately 10 to 15 mm, so that already small eccentricities can cause relatively large measuring errors. Therefore, the required maximum acceptable eccentricities of 0.02 mm often cannot be realized even with the above described very complicated boring apparatus.